Grade 8 – Lesson 10.3 Absolute-Age Dating Essential Questions 1. What does absolute age mean? 2. How can radioactive decay be used to date rocks? New Vocabulary English Word Arabic Word English Definition Absolute age Isotope Radioactive decay Half-Life Carbon Dating Absolute ages of rocks Scientists use the term absolute age to mean the numerical age, in years, of a rock or object. By measuring the absolute ages of rocks, geologists have developed accurate historical records for many geologic formations. Scientists have been able to determine the absolute ages of rocks only since the beginning of the twentieth century. That is when radioactivity was discovered. Radioactivity is the release of energy from unstable atoms. Atoms An atom is the smallest part of an element. Each atom contains smaller particles called protons, neutrons, and electrons. Protons and neutrons are in an atom’s nucleus. Electrons surround the nucleus. Isotopes Isotopes are atoms of the same element that have different numbers of neutrons. Radioactive Decay Some isotopes are unstable. These isotopes are known as radioactive isotopes. Radioactive isotopes decay, or change, over time. As they decay, they release energy and form new, stable atoms. Radioactive decay is the process by which an unstable element naturally changes into another element that is stable. Half-Life The rate of radioactive decay is constant for a given element. This rate is measured in time units called half-lives. An isotope’s half-life is the time required for half of the parent isotopes to decay into daughter isotopes. Half-lives of radioactive isotopes range from a few microseconds to billions of years. When half the parent isotopes have decayed into daughter isotopes, the isotope has reached one half-life. At this point, 50 percent of the isotopes are parents and 50 percent of the isotopes are daughters. After two half-lives, one-half of the remaining parent isotopes have decayed so that only one-quarter as much parent remains as at the start. At this point, 25 percent of the isotopes are parent and 75 percent of the isotopes are daughter. This process continues until nearly all parent isotopes have decayed into daughter isotopes. Radiometric Ages Because radioactive isotopes decay at a constant rate, they can be used like clocks to measure the age of the material that contains them. Radiocarbon Dating One important radioactive isotope used for dating is an isotope of carbon called radiocarbon. Radiocarbon is also known as carbon-14, or C-14, because there are 14 particles in its nucleus—six protons and eight neutrons. As long as an organism is alive, the ratio of C-14 to C-12 in its tissues remain constant. However, when an organism dies the C14 present in the organism starts to decay to nitrogen-14 (N-14). As the dead organism’s C14 decays, the ratio of C-14 to C-12 changes. Scientists measure the ratio of C-14 to C-12 in the remains of the dead organism to determine how much time has passed since the organism died. The half-life of carbon-14 is 5,730 years and is therefore not very useful to date any organism older than 60 000 years. Dating Rocks Radiocarbon dating is useful only for dating organic material—material from once-living organisms. Most rocks do not contain organic material. For dating rocks, geologists use different kinds of radioactive isotopes. Dating Igneous Rock One of the most common isotopes used in radiometric dating is uranium-235, or U-235. U235 is often trapped in the minerals of igneous rocks that crystallize from hot, molten magma. As soon as U-235 is trapped in a mineral, it begins to decay to lead-207, or Pb-207, Different Types of Isotopes The half-life of uranium-235 is 704 million years. This makes it useful for dating rocks that are very old. Radioactive isotopes with short half-lives cannot be used for dating old rocks. They do not contain enough parent isotope to measure. Geologists often use a combination of radioactive isotopes to measure the age of a rock. Radiometric dating of rocks from the Moon and meteorites indicate that Earth is 4.54 billion years old. Scientists accept this age because evidence suggests that Earth, the Moon, and meteorites all formed at about the same time. Radiometric dating, the relative order of rock layers, and fossils all help scientists understand Earth’s long history.
